1. Field of the Invention
The present invention pertains to MOS oscillators suitable for VLSI structures. More particularly, the present invention pertains to adiabatic MOS oscillator operation.
2. Discussion of Related Art
Personal communication system (PCS) devices such as cellular phones are playing an increasingly important role in the consumer electronics marketplace. Structures using MOS and CMOS technology are advantageous for such circuits because of their small size and low power consumption characteristics.
MOS ring oscillators are well known. However, their capacitive loading, and the resulting power dissipation and thermal instability, are unattractive. Also, the operating frequency of these oscillators is set by resistively varying the bias voltage applied to each inverter in the ring. However, as is well known, resistively varying the frequency of oscillators causes their power consumption to increase as their output frequency increases.
Conventional MOS ring oscillators also require an inverter string at the output in order to scale up the capacitance needed to maintain oscillation in the ring to the larger capacitance of the ouput driver transistor needed to isolate the ring from relatively large impedances, such as 50 ohm transmission lines. However, the parasitic gate capacitances of the inverter strings themselves add substantially to the load on the oscillator, causing the strings to dissipate substantial power before the oscillator output is delivered to the load, as much as one-third of the power delivered by the ring.
The power dissipation occurring in the output strings of ring oscillators is avoided entirely when the MOS oscillator is implemented using a tank circuit, which uses the capacitance to maintain its oscillation. However, this requires the integration of passive elements into the circuit.
Passive elements have been formed in integrated circuits. N. Nguyen and R. Meyer, "Si IC-Compatible Inductors and compatible LC Passive Filters", IEEE Journal Solid State Circuits, vol. 25, no. 4, pp. 1028-1031, August 1990, describes bipolar circuits that include integrated inductors. The process used for fabricating bipolar circuits is, in general, not compatible with CMOS fabrication. Also, because of their characteristic power constraints, bipolar devices require more "real estate" on the chip, generate substantially more heat and lack the low-power performance capability of CMOS devices. Therefore they are less suitable for VLSI structures, generally.
J. Chang, A. Abidi and M. Gaitan, "Large Suspended Inductors on Silicon and their use in a 2-micron CMOS Rf Amplifier", IEEE Electronic Device Letters, vol. 14, no. 5, pp. 246-248, May 1993, discloses an integrated tank structure for CMOS where the silicon under a glass passivation layer that supports the inductor metalization is etched away to reduce power losses in the integrated inductor. This complicates device fabrication, adding an extra step. Also, in their manufacture, the actual resonant frequency of integrated tank circuits vary. To operate at a given frequency, these tank circuit must be resistively tuned and, consequently, they are inefficient.
Small size and relatively low power consumption make CMOS potentially advantageous for use in cellular telephones and other personal communication systems (PCSs). However, highly-doped substrates used in CMOS fabrication would cause unacceptably high power loss if the integrated inductors used in bipolar circuits were implemented in MOS structures, and the high cost of integrating passive devices into MOS circuit fabrication, even in lightly-doped structures, and the power loss encountered in capacitance-matching for ring oscillators and in tuning CMOS oscillators, generally, has made their use in PCS devices problematic.